This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Measuring the unfolding of a protein in response to increasing concentrations of chemical denaturants can be used to derive its thermodynamic stability. T7 phage are stable in relatively high concentrations of denaturant. We have exploited this fact to perform denaturation experiments on mutants of a WW domain on a massive scale. We exposed a library of phage-displayed WW domain variants to increasing concentrations of the chemical denaturant guanidine and then performed a selection for peptide binding. By examining the performance of each variant in increasing concentrations of guanidine, we can calculate the relative thermodynamic stability of each variant. The resulting stability map of the WW domain should offer an unprecedented view of regions governing the stability of this domain and may illuminate general principles of protein stability.